ABSTRACT: Nucleotide bases are ubiquitous in living organisms, but the fate of these compounds in natural environments is poorly understood. Here we studied the metabolism of selected purines and pyrimidines in estuarine bacterial assemblages from the Øresund, Denmark. Depletion of nucleotide bases in the incubations was followed by the appearance of urea. The purine guanine and the catabolic intermediates hypoxanthine and xanthine were depleted 2 times faster from seawater incubations than the purine adenine, and 8 to 35 times faster than the pyrimidines thymine, cytosine, and uracil over the course of 141 h. After 48 h, 45 to 60% of guanine-N, hypoxanthine-N and xanthine-N was converted to urea-N while the conversion of adenine and pyrimidines to urea was slower, corresponding to 34% and 19 to 23%, respectively. After 96 h, urea concentrations declined in most of the incubations, indicating hydrolysis of urea by the bacterial populations. Bacterial metabolism of adenine in Øresund water was estimated to contribute up to 10% of the urea pool, but due to the efficient conversion of adenine, the ecosystem importance of adenine degradation to urea production was most likely greater. Growth of bacterial microcolonies on 0.2 µm pore-sized polycarbonate filters floating on natural seawater enriched with individual nucleotide bases varied significantly with substrate enrichment. Despite growth of only a small fraction of bacteria present in the natural assemblage on the filters, variation in bacterial microcolony biomass explained most of the variation in substrate utilization and urea production among the treatments. Our results suggest that bacterial catabolism of particularly purines and their intermediates, and to lesser extent pyrimidines, is a major process by which the N moiety of natural, heterocyclic bases are converted into urea which is easily assimilated.